The present invention relates to a fluid flowmeter, more particularly to a fluid flowmeter with a first acoustic transducer located upstream in relation to a second acoustic transducer, where the time of flight of acoustic waves between the transducers is used to measure the flow velocity of a fluid medium which flows between them.
In WO 96/06333, a flowmeter is shown where two concentric pipes form an annular fluid flow passage which allows a medium to enter the inner pipe up to a central element that prevents further flow. The acoustic signals are absorbed by or reflected from the central element.
WO 94/09342, shows a flowmeter that uses plain acoustic waves where the flow path is divided into a plurality of parallel passages which are dimensioned such that the characteristic frequency of the plain wave is higher than the transmission frequency.
WO 94/17372 shows a fluid flowmeter which uses a flow structure located between two transducers. The flow structure is defined by an array of fluid flow passages or an annular fluid flow passage. This fluid flowmeter works most effectively only when plain acoustic waves propagate between the transducers. This can be ensured, in the case of cylindrical fluid flow passages, if the wave length of the sound transmitted is greater than d/0.568 where d is the diameter of the flow passage. The problem which can arise in this situation is that this relatively small diameter can give rise to unwanted effects such as high pressure loss and unwanted acoustic phase shifts at the entry to and the exit from the fluid flow passage. However, if the size of the diameter is increased, higher order modes are excited in the fluid flow passage and these result in errors in the meter. In particular, if a flow structure comprising an annular array or ring of passages is used together with a centrally positioned, coaxial transducer, then asymmetric acoustic modes are produced.
The present invention contemplates a new and improved apparatus and method which overcomes the above mentioned problems.
In accordance with the present invention, there is provided a flowmeter where the fluid flow passage has at least one attenuation structure positioned so as to attenuate at least one asymmetric acoustic propagation mode. The attenuation structure extends substantially over the length of the fluid flow passage and is positioned on the circumference of the fluid flow passage to correspond with the location of the anti-node of at least one asymmetric acoustic propagation mode.
In accordance with another aspect of the invention, wider flow passages are used to avoid the problems of asymmetric acoustic propagation modes by suitably positioning an attenuation means. This allows the plain wave mode to dominate the other modes and reduces the effects of the other modes on the resultant fluid flow value which is obtained. The attenuation structure runs parallel to the fluid flow passage axis.
In accordance with still another aspect of the invention, an asymmetric propagation mode has an asymmetric distribution around the circumference of the fluid flow passage. This distribution helps to define wave nodes and wave anti-nodes which represent points of a relatively low or high energy of the asymmetric propagation mode, and maximizes the effect of the attenuation structure. Although the attenuation structure has a certain effect on the propagation plane wave, it has a much greater effect on the asymmetric propagation mode by increasing the energy of the plane waves arriving at the receiver relative to the energy from unwanted modes.
Known attenuation structures have a layer of attenuating material provided along the fluid flow passage, but in many cases this will lead to problems of turbulence and the like. Therefore, according to still another aspect of the invention, the attenuation structure comprises an opening facing into the fluid passageway and extending into or through the wall of the fluid flow passage where either one or more openings will be located on one side or a number of openings will be provided on opposite sides of the fluid flow passage corresponding to respective anti-nodes of the asymmetric acoustic propagation mode. These openings are either slots, holes, or a series of holes and the openings either go through the wall or end as blind openings in the wall.
In accordance with yet another aspect of the invention, the attenuation of the at least one asymmetric acoustic propagation mode relative to the plane wave is improved by providing a sound absorbent material within or at a laterally outer end of the opening.
In accordance with a further aspect of the invention, a laterally outer end of the opening, where the walls of the opening could be covered with a material, have a multiplicity of the wall cavities facing toward the opening entrance. These cause the viscose losses in the fluid to be high and highly attenuate the at least one asymmetric acoustic propagation mode. The plane wave is hardly affected by this material. For example, this material may be a gritted material, i.e., sandpaper.
Although the invention has been described in relation to a flow structure having a single fluid flow passage, in accordance with another embodiment of the invention, the flowmeter comprises an array of fluid flow passages, typically an annular array. In this embodiment the passages are arranged symmetrically with respect to the transducers. In one preferred arrangement, each passage has an opening extending along it. Each opening faces into the respective fluid flow passage and is positioned at a radially inward position of the respective fluid flow passage. The opening extends into or through the wall of the fluid flow passage. Each opening may be blind or be in communication with a common, internal passage.
One advantage of this arrangement is the constructional advantage.
A second advantage is an increased attenuation effect of the individual openings where traditionally, the common, internal passage will have an annular shape.
In accordance with a still further aspect of the invention, the meter includes an array, typically an annular array, of fluid flow passages arranged symmetrically with respect to the transducers, wherein each passage has an opening extending along it. Each opening facing into the respective fluid flow passage is positioned at a radially outward position of the respective fluid flow passage and extends into or through the wall of the fluid flow passage. Furthermore, both inner and outer openings could be provided. Typically, each passage in an array of fluid flow passages will have an identical form. A cross-section of each fluid flow passage can be circular, elliptical, rectangular, or hexagonal. Finally, a single, annular fluid flow passage could also be used according to the invention.
It is to be appreciated that although the invention is applicable to the metering of any fluid, including liquids, it is particularly useful for metering gas and it is highly suitable for domestic gas metering.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.